1. Field of the Invention
This invention relates to a semiconductor integrated circuit, and more particularly to a semiconductor integrated circuit having a CMOS crystal oscillator circuit and a frequency dividing circuit formed on the same semiconductor substrate.
2. Description of the Related Art
As shown in FIG. 1, a crystal oscillator circuit having a CMOS crystal oscillator circuit 1 and a frequency dividing circuit 9 for dividing the frequency of the crystal oscillation output formed on the same semiconductor substrate selects a signal having one of the original oscillation frequency fo and a divided frequency fo/2n and outputs the selected signal as an output in the prior art. In FIG. 1, a reference numeral 10 denotes a decoder, 11 an output circuit.
FIG. 2 shows an example of the circuit. In this example, the original oscillation circuit 1 is formed of a CMOS inverter 2, a feedback resistor 3 and an external circuit 7 which is constructed by a crystal oscillator 4 and pull-down capacitors 5 and 6. The original oscillation signal (frequency fo) of the circuit is input to a clocked inverter CI1 via a CMOS inverter 8 and to a frequency divider 9 (which is formed with a three stage construction in this example) via the inverter and signals of frequencies fo/2, fo/4 and fo/8 are output from the frequency divider 9. A desired one of the signals respectively having the original oscillation frequency fo, divided frequencies fo/2, fo/4 and fo/8 may be selected by inputting control signals A and B to the decoder 10, activating one of outputs C, D, E and F thereof and setting one of the clocked inverters CI1 to CI4 into the operative state by the activated output. The selected signal is derived from an output section via the 3-state output circuit 11. The potential of an oscillation interrupting terminal 13 is generally set to an "H (high)" level or power source level V.sub.DD by means of a pull-up resistor 14. If a signal of "L (low)" level or ground level is supplied to the oscillation interrupting terminal 13, the low level signal is supplied to an N-channel MOS transistor 16 for oscillation interruption via a CMOS inverter 15 to turn on the transistor 16 and set the potential of a node 17 to the "L" level, thereby interrupting the oscillation of the oscillation circuit 1. Further, the signal on the terminal 13 is input to a reset terminal R of the frequency divider 9 via the CMOS inverters 15 and 18 to interrupt the frequency dividing operation of the frequency divider 9. The signal is also input to a control terminal Co of the 3-state output circuit 11 via the CMOS inverters 15 and 18 to set the 3-state output circuit 11 to a high impedance state.
Since the frequency dividing circuit 9 is operated even when the oscillation output is selected by the clocked inverter CI1 in the circuit of FIG. 2, noise contained in the oscillation output is increased by the switching noise generated from the frequency dividing circuit 9. The reason is that the power source current is varied in the switching operation caused during the frequency dividing operation of the frequency dividing H circuit 9 and the power source V.sub.DD tends to be influenced by the generated noise. Particularly, in the integrated circuit, since the frequency dividing circuit 9, oscillation circuit 1 and inverter 8 connected between the frequency dividing circuit 9 and the oscillation circuit 1 commonly use the power source V.sub.DD, the power source noise generated from the frequency dividing circuit 9 may influence the power source voltage of the oscillation circuit 1 and the like and the noise may be superposed on an oscillation signal output via the inverter 8, clocked inverter CI1 and output circuit 11. However, since the frequency dividing circuit 9 itself has a wave forming function, the noise contained in the output of the frequency dividing circuit 9 is extremely small.